Journal of Tropical Ecology (2010) 26:35–43. Copyright © Cambridge University Press 2009 doi:10.1017/S0266467409990435 Forest fragmentation reduces recruitment of large-seeded tree species in a semi-deciduous tropical forest of southern Mexico Felipe P. L. Melo ∗,1 , Esteban Mart´ ınez-Salas†, Julieta Ben´ ıtez-Malvido‡ and Gerardo Ceballos ∗ ∗ Instituto de Ecolog´ ıa, Universidad Nacional Aut ´ onoma de M ´ exico. Apartado Postal 70-275, C.P. 04510, Ciudad Universitaria, M ´ exico DF, M ´ exico † Instituto de Biolog´ ıa, Universidad Nacional Aut ´ onoma de M ´ exico. Apartado Postal 70-367, C.P. 04510, Ciudad Universitaria, M ´ exico DF, M ´ exico ‡ Centro de Investigaciones en Ecosistemas, Universidad Nacional Aut ´ onoma de M ´ exico. Antigua Carretera a P ´ atzcuaro no. 8701, Ex-Hacienda de San Jos ´ e de la Huerta, C.P. 59180, Morelia, Michoac ´ an, M ´ exico (Accepted 19 September 2009) Abstract: This study tested whether the reduction in the number of large-bodied seed dispersers is correlated with shifts in the taxonomic and functional (e.g. dispersal mode and seed size) traits of the seeding communities within small fragments of semi-deciduous forest, southern Mexico. In five fragments (2.3–640 ha) and one continuous forest site we sampled tree and seedling species in 40 (20 × 20 m) and 120 (3 × 3 m) plots respectively, and recorded the incidence (presence/absence) of the disperser fauna (three common large-birds and >500-g mammals). Tree and seedling species were categorized according to dispersal mode, seed size and whether they originated from local (i.e. from dropped) or immigrant (i.e. from actively dispersed) seeds. Fragment size negatively correlated with number of species of medium to large vertebrate seed-dispersers and number of seedlings of large-seeded species, but had no influence on functional traits of the adult-tree community. Between 41% and 61% of all seedlings were considered as immigrants and the proportion of immigrant seedlings of large-seeded tree species was negatively correlated with forest size. The results suggest that biased defaunation in small forest fragments may seriously reduce recruitment of large-seeded tree species (>1.4 cm length) dispersed by vertebrates, negatively affecting successional trajectories of small forest fragments. Key Words: defaunation, fragmentation, regeneration, seed dispersal, seedling recruitment, tropical forest INTRODUCTION Fragmentation of tropical forests is increasing worldwide along with annual deforestation rates of about 5.8 million ha (Achard et al. 2002). Together with habitat loss, forest fragmentation facilitates access to hunters who strongly deplete vertebrate populations (Corlett 2007). Number of vertebrate species in forest fragments is expected to be strongly area-dependent (Chiarello 1999, Peres & Michalski 2006), though degree of fragment isolation, matrix type and time since isolation should also be determinant (Andr´ en 1994). In extreme cases, highly fragmented landscapes have lost virtually all large vertebrate species (>1 kg) which are assumed to be regionally extinct over thousands of square kilometres (Silva & Pontes 2008). 1 Corresponding author. Current address: Centro de Pesquisas Ambientais do Nordeste. Av Caxang ´ a, 5775, sala 05, CEP: 50740–000, Recife, Pernambuco, Brasil. Email: fplmelo@cepan.org.br Most tropical tree species are dispersed by animals (Gentry 1982, Howe & Smallwood 1982) and depend on successful seed dispersal to enhance seedling recruitment and allow forest regeneration (Stoner et al. 2007). Defaunation reduces the dispersal of zoochorous tree species (Cordeiro & Howe 2001, Melo et al. 2006) and alters seed predation patterns through elimination of granivores with perceivable consequences to plant demography (Terborgh et al. 2001). However, controversial explanations on the effects of defaunation on forest regeneration still persist. There is no consensus, for example, on whether the lack of large-bodied granivores favours large-seeded species by an ecological release from seed predators (Wright et al. 2007) or whether the low seed dispersal due to lack of large-bodied seed dispersers negatively affect large-seeded species (Silva & Tabarelli 2000). Such a divergence has led to opposite predictions on the consequences of defaunation on plant species composition in fragmented landscapes (see Cramer et al. 2007 and Dirzo et al. 2007 for contrasting conclusions).